Modulated carrier wave transmitter



Aug. 18, 1936.

F. c.LUNNoN er nu.l v2,051,493

' MODULATED CARRIER WAVE TRANSMITTER Filed July V2', 1933 ATTORNEY Patented Aug. 18, 1936 UNITED STATES PATENT OFFICE MODULATED CARRIER WAVE TRANSMITTER Frederick Charles Lunnon and Edward Herbert Trump, Chelmsford,

England, assignors to Radio Corporation of America, a corporation of Delaware 8 Claims.

'I'his invention relates to radio and other modulated carrier wave transmitters.

In the well known commonly employed socalled grid direct current modulation circuit arrangements, i. e., arrangements wherein the discharge space of a modulation tube to which modulation potentials are applied is included in the direct current grid circuit of the main tube, i. e., the high frequency tube to be modulated, the amplitude of the driving voltage applied to the main tube is fixed and the grid bias thereof is varied at modulating frequency, the anode voltage remaining constant. This grid bias variation is, however, not controlled directly by the modulating voltage but indirectly by modulation of the grid leak resistance in the grid circuit of said main tube and therefore the resulting bias, the product of grid current times grid leak resistance, will depend upon the grid current characteristic of said main tube7 i. e., upon the shape of the curve produced by plotting grid current, as measured by a direct current instrument as ordinates, against grid bias voltage as abscissa when the driving voltage remains constant.

The degree of modulation obtainable by varying the grid leak resistance depends on the relative values of the minimum and maximum obtainable anode input to the tube of the modulated stage. By minimum and maximum obtainable anode input is meant the range through which the anode current drawn by the modulated stage may be varied by the modulating potential variations.

The minimum anode input is obtained when the leak resistance has its maximum resistance value in which case the grid bias of the modulated tube will be approximately equal to the peak value of the driving voltage and the grid of the modulated tube will then be driven only Very slightly positive.

The maximum anode input is obtained when the grid leak resistance has its minimum value, this value of resistance being that which would be used for a fixed value of grid lea resistance if the transmitter were being used as a telegraph transmitter. In this case the grid of the modulated tube will be driven well positive at the anode energy of the modulating tube. The degree of modulation Will be small if the minimum anode 50 input is a large proportion of the maximum anode input and this state of affairs may arise from either or both of two causes, namely:

(l) The use of an amplifier tube of low amplification operating at an anode voltage such that the anode input is high when the grid is driven just positive and;

(2) The use of an amplifier tube having sufficient secondary grid emission to cause the direct current grid current to fall to zero while the grid 5 is being driven well positive.

In practice the second cause is usually the most serious and may easily be so serious that practically no Variation of anode input can be obtained by increasing the grid leak resistance. 10 What usually happens in such cases is that the anode input is reduced only very slightly as the grid leak resistance is increased to quite high values and then suddenly it falls to a 10W value in a discontinuous manner; in extreme cases the 15 grid current may reappear positive or negative after first falling to zero as the grid leak resistance is still further increased, so that the grid bias, i. e., the product of grid leak resistance and grid current, does not increase steadily with increasing grid leak resistance and this is reiiected in the anode input.

The net result is that the direct current grid method of modulation, in its simplest form, can not be used with an amplier tube having such characteristics.

These diiculties, which are known, may be obviated by providing a separate source of direct current potential in series with a resistance across the variable grid leak of the amplier tube. In practice, of course, this Variable grid leak is constituted by the anode-cathode resistance of a triode to whose `grid the modulating voltages are applied. The series resistance is chosen at some convenient mean value between the maximum and minimum values of the resistance of the modulating tube corresponding respectively with the negative and positive half-cycle peaks of modulating voltage. With such an arrangement when the modulating tube resistance is almost iniinite the full potential of the direct current source is applied to the amplier grid and the amplitude of this potential is chosen such that the amplifier anode input isreduced to zero. When the modulating tube resistance is at its lowest value, the direct current potential applied to the amplifier grid from the direct current source is almost negligible, as the fixed resistance has then a value of some ten times or more of the resistance of the modulating tube. Under this particular condition, therefore, the bias of the amplifier grid is determined almost entirely by the Values of the grid current Vand of the modulating tube resistance. At this end of the modulation characteristic the amplifier grid is specification being driven heavily positive and, therefore, secondary emission troubles are unimportant.

The conditions which obtain at both extremes of resistance of the modulating tube have now been explained, and for all intermediate values it is sufficient to say that in practice the balance` between bias derived from the product of grid current andmodulating tube resistance and bias derived from the separate direct current source gradually changes over as the modulating tube resistance changes, and the final effect is that of a modulation characteristic of satisfactory form and substantially rectilinear over a desired range is obtained whatever may be the grid and anode charactersticsof the modulated amplifier tube.

ForV further information as regards a modified and improved direct current grid modulation system as just described, reference is directed to the accompanying the co-pending British application No. 2,286/32. See also Lunnon et al, U. S. patent application No. 623,750,

July 21, 1932.

Our previous British Patent No. 351,051 described aso-called absorber keying system in which there isutilized as an absorber tube dur- Ying spacing periods, a tube which during marking periods functions Vas an amplifier. The system shown'in British Patent No. 351,051 has also been` disclosed-in Lunnons U. S. application No. 520,587, March 6, 1931, U. S. Patent No. r1,938,631, issued Dec. 12, V1933. For the purpose of brief descriptionV in the present specification such an absorber system lwill be herein referred to as an auto absorber system. Y

The present invention may be regarded as providing anarrangement wherein the advantages ofV both the autoabsorber systems and the ima proved direct current grid modulation systems hereinbefore referred tomay be secured in a sin- Y gle transmitter, or to put the matter in another Way, the present invention may be regarded as avoiding ,a difliculty which arises when both the Y said systems hereinbefore' described are incorporatedv in a single transmitter.

In order that the Vdiflicultywliich the present invention avoids may be clearly understood, consider the c ase'of a transmitterV incorporating both the systems hereinbefore described. Such a Vtransmitter would, as shown inthe accompanying Figure 1, comprise anamplifier tube I having anv earthed cathode and to whose grid circuit the V high frequency oscillations to be modulated are applied, theV grid 2 of this tube being connected throughv a grid coil or choke 3'to the cathode 4 of avmodulating tube 5, to whose grid circuit specchi-voltages, for example, are applied, e; g., through a transformer T.

The grid choke 3serves to maintain the high `frequency driving voltage on the grid 2 of the valve I and in conjunction with the bi-pass condenser 'I to :prevent the application of high fre- Y quency driving voltage to the modulating tube k5 and associated'apparatus.

The lower endof the gridrchoke 3 is connected tothe cathode Siof the tube I through a bi-pass condenser V'I and the cathode 4 is connected to the anode 8 ofthe modulating tube 5 through a. resistance 9 and a source. of potential I0 in series, the negative terminal of this source being to- Wards the cathode. The anode 8 of the modulating` tube 5l is connected toV a tapping point IIV uponV a potentiometer resistance I2 which is connected at oneend'to earth and at the other end through a'further source of potential I3, and the Yanode cathode space'of' a` keying or control tube Vof a resistance I5 shunted by a condenser I6 and rectified speech voltages are applied to the grid circuit of the said keying or control tube across the time control circuit. Further tapping points I1,V I8 upon the potentiometer resistance I2 are connected to the grids of the tubes in stages (not shown) of the transmitter preceding the magnifier valve I at which modulation is effected. The potentiometer resistance with its associated tapping points and connections are designed to enable auto-absorber keying to be effected generally as set forth in our prior U. S. application No. 520,587, U. S. Patent No. 1,938,631, so that in the absence of speech there will be no rectified currents through the resistance I5 in the time control circuit, the grid of the keying or control tube I4 will be at cathode potential and the current flowing through the potentiometer resistblocked and the current -through the potentiometer I2 accordingly reduced -to-zero, so that the transmitter is changed over rto the ,'marking condition simultaneously with the arrival of speech voltages at the tube. In this manner sufficient current` is absorbed during no signals to regulate the load ,on the anode potential source. control of the amplifier tube I is concerned, the difficulty arises that-the auto-absorber Vaction is nulliiied due to the presence of thersource of potential I0.V This source of potential is rnormally greatly in excess of and is additive to the bias derived from the potentiometer resistance I2. Therefore, vwith the arrangement illustrated in Figure 1, the Vamplifier valve I, which is generally the final amplifier in the transmitter, or at any rate is late. in the chain of amplification at the transmitter, would be biased to the cut-off cond1tion at spacing, and this isa serious disadvantage since, of course, if the tube I is the final grid of the modulating Now, so far as 'theVV iliary source of potential is connected in the grid circuit of the Vtube at which modulation islto beV rec-Y eifected, this auxiliary voltage is obtained by tifying high frequency voltage derived from some convenent stage in the transmitter, preferably from the output of the amplifier stage preceding the stage at which modulation is effected. In this way the auxiliary voltage which may be referred to as the modulation correction voltage, F

is reduced to zero during spacing, since during spacing there will be no high frequency output from the penultimate amplifier stage, assuming the rectifier voltage toV be derived from this stage, and accordingly the bias applied to the final amplier stage, at which modulation is effected, will be that derived from the potentiometer resistance and therefore the said final amplifier stage will take its correct no load current during spac- One arrangement in accordance with this invention will now be described with reference to the accompanying Figure 2. In the arrangement now to be described, the modulation is effected at the final amplifier stage of the transmitter, which stage consists of two tubes Ia, Ib, symmetrically arranged. The grids of the two tubes are connected through the usual coupling condensers I9, 20 to the output terminals of the preceding amplifier, the said terminals being also connected preferably through variable coupling condensers 2I, 22 to the plates 23, 2&1y of a pair of high frequency rectifiers, e. g., diodes 25, 25. The plates 23, 24 are connected together through a coil 21 and the center point 28 of this coil is connected through a resistance 29 shunted by a smoothing condenser 31 to the plate of the tube 5 and also to .the midpoint-s of two resistances 38, 35, each of which is connected across the cathode of one of the diodes. The direct current output from the diodes may be varied if desired by changing the condensers 2| and 22 in capacity. A similar adjustment can be obtained by utilizing the resistance 29 as a potentiometer as indicated in broken lines in Figure 2, i. e., by connecting the anode of the tube 5 to an adjustable tapping point on resistance 29 instead of, as shown in full lines, to the lower end thereof.

The cathodes of the diodes are heated by alternating current through separate transformers, as shown, the secondary of one transformer being connected across the cathode of one diode and the secondary of theother being connected in series with a reactive impedance 32 across the cathode of the other diode. yThis arrangement has the advantage that the alternating currents heating the two diode cathodes are not in phase with one another and therefore variations of cathode temperature, due to alternating current heating do not occur in phase in the two diodes so that, as a result, constant modulation of the emitted carrier wave which might otherwise occur due to alternating current cathode heating in the diodes is avoided. A similar result may be achieved by heating the cathodes of the two diodes each from a different phase of a multi-phase supply.

The variable coupling condensers 2i, 22 serve to regulate the amplitude of the rectifier voltage obtained from the diodes while the resistance 29 serves to provide a continuous load upon the diodes and thus to prevent voltage uctuations as the resistance of the modulation tube varies in sympathy with the speech voltages applied to the grid. The grids of the tubes Ia, Ib are connected together through the grid coils 33, 35, the junction point of which is connected through a condenser to the common earthed point of the cathodes of the tubes Ia and Ib, said junction point being also connected to the cathode of the modulation tube 5. The cathode 'of this tube is connected through a resistance 36 to the midpoint of the inductance 21. Speech voltages are applied as before to the grid circuit of the modulation tube, and the anode 8 of this tube is connected to a tapping point II on a potentiometer resistance I2 in series with a source of potential I3 and the keying tube I4. Also, as before, the keying tube has a time control circuit I5, I6 connected between its grid and cathode and receiver rectied speech voltages, further tappings I1,

. I8 upon the potentiometer resistance I2 being taken to the grids of preceding stages in the transmitter. It will be seen that with this arrangement, modulation correction direct current voltage is derived from the rectiers or diodes in dependence upon the high frequency output from' the penultimate amplifier stage, and accordingly at spacing, when there is no high frequency output from this stage, there will be no modulation correction voltage, so that the total bias received by the iinal amplifier stage will be that obtained from the potentiometer resistance.

In operation high frequency oscillations are developed in the source of oscillations and amplified and applied by way of condensers I9 and 2D to the grids of the amplifiers Ia and Ib and from the anodes of these amplifiers to a load circuit. Radio frequency potentials are also supplied to the input electrodes of rectifiers 25 and 26, the outputs of which rectifiers are in series with the anode to cathode impedance of tube 5. The impedance of tube 5 is the grid leak resistance of the stage Ia and Ib in which modulation is to take place. When no modulating potentials are applied tube I4 is conductiveand the potential drop through resistance I2 is applied to one or more of the early stages to bias the same to cutoff to stop transmission. In order to regulate the load on the anode source some of the later stages are caused to draw a certain amount of current even though the transmitter is in general in no load condition. Now the final stage usually includes higher power tubes and therefore should furnish a large part of this no load current, but heretofore due to the fact that the grid leak impedance (high impedance between anode and cathode of tube 5, Figure 1) is high at this time, the source I0 applies a high negative potential to tube I to completely cut off anode current thereto. In our improvement, however, I0 has been replaced by the rectiers 25 and 26 which draw their energy from the radio frequency stages and the amplitude of the energy so drawn'and the operating elements of the rectifiers are so adjusted that the final stage draws its share of no load current. Since at `spacing or no modulation no energy is applied to the rectiiiers 25 and 26 and there will be no modulation correction voltage applied from said rectiiiers to the tube 5 and tothe grid circuit of the tubes la, and Ib.

When modulating potentials are applied, current flows through resistance I5, tube I4 becomes non-conductive, the potentials at the points along resistance I2 become more positive and the preceding stages in the transmitter become operative and a carrier wave is applied to the amplifiers la and Ib` and to the rectiers 25 and 25 which supply correcting current to the modulation circuits.

Having thus described our invention and the operation thereof, what we claim is:

1. In a signalling system, a source of carrier frequency oscillations, a thermionic amplifier tube having an anode, a cathode, and a control grid, a source of modulating potentials, a thermionic modulating tube having anode, cathode and con- 6' trol grid electrodes and having its control grid and cathode electrodes coupled to said source of modulating potentials and its anode and cathode electrodes connected in series with the control grid 'and cathode electrodes of said thermionic amplifier to modulate therein the carrier waves applied thereto when direct current potentials are applied between the anode and cathode and control grid and cathode of said tubes, and means for correcting the direct current potentials applied rbetween'the` controlgridsandcathode of said ,thermionic amplifier comprising a rectifier having its input electrodes variablycoupled to said source Vof highv frequency oscillations andv its Voutput electrodes connected in series with the input electrodes to said thermionic modulator tube.

2. Atransmitting device comprising, a source ofhigh frequency oscillations, a pair of thermionic amplifier tubes, each of said tubes having an anode, .a cathode and. a control grid, circuits coupling kthe outputV of saidfsource of high fre- Y quency oscillations to the control grids of said thermionic :amplifier tubes, asource of Ymodulating potentials, an additional thermionic tube having an anode, a cathode and a control grid, a connection between said source of modulating potentials andlthe control grid and cathode of o Y saidl additional tube, a connection between the Vanode and cathode of said additional tube and the control gridsand cathodes of said thermionic amplifiers, a source of biasing potential for said source of oscillations, said source of biasing p0- tentials comprising a thermionic rectifier having its input electrodes connected With said source of modulating potentials and its output electrodes Y coupled to a potentiometena point on which is Aconnected to saidfsource of'oscillations Yandina VAthermionic rectifier having its input electrodes coupled to said source of Voscillations and its output electrodes connected with the anode and cathode of said additional tube. l

3. Atransmitting device comprising, a source Yof'high frequency oscillations, a pair of thermionic Yamplifier tubes, each of said tubes having an anode,-a cathode and a control grid, a high frequencyV amplifier having its input electrodes coupled to said source of high frequency oscillations and its output electrodes VVcoupled tofthe control grids of said thermionic amplifier tubes, a source of modulating potentials, an additional thermionic tube having an anode, a cathode'and za-.control grid, a connection between saidrsource of modulating potentials and the control grid and cathode of said additional tube, a connection f between the anode and cathode of saidfadditional tube and thecontrol grids'and cathodes of said pair of thermionic amplifier tubes, said connection including a portion of a tapped resistance, a source of biasing-potential for said source of 0scillations and for said' high frequency amplifier, said source of biasing potentials vcomprising a .thermionic rectifier'having its input electrodes connected with said source of modulating potentials and its output electrodes coupled to said resistance, points on which are connected to said source-of oscillations and to said high frequency amplifier and a thermionic rectifier having its input electrodes coupled to said high frequencyl amplifier and its output electrodes connected in series with the anode and cathode of said additional tube.

4. A transmitting devicerco'mprising, a' source high frequency oscillations, a pair of thermionic amplifier tubes, each of said tubes having an anode; a cathode and a control grid, an additional amplifier having its input electrodes coupled to o Vsaid source of high frequency oscillations and its output electrodes coupled to the control grids of saidthermionic amplifier tubes, a source of modulating. potentials,s a modulating ktube having an l anode, a cathode and acontrol grid, a connection between said source of modulating potentials and the controligrid. and cathodeof said modulating tube, a connection-between the anode and 1 cathodezof said modulating tube and the control grids and cathodes of said thermionic amplifier tubes-a source of biasing potentialfor said source of oscillations and for said Vadditional amplier,

said Source of biasing potentials comprising af resistance'points of which are connected to said source of oscillations and to said additional amplifier and a thermionic rectifier having its input electrodes coupled to said additional amplifier .and its output electrodes connected with the =L f anode and cathode of said modulating tube.

5. A signalling Vsystem comprising, arsource of oscillations, athermionic amplifier tube having a control grid and a cathode, a connection between said source of oscillations and the control* Vgrid of said amplifier tube,.a source of modulating potentials, a thermionic modulating tube having its control grid electrode coupled to said Vsource ofV modulating potentials and its cathode electrode coupled to the control grid in said thermionic amplierand its'anode coupled to the cathode of said amplifier tube to modulate therein the carrier waves'applied thereto whenthe electrodes of said tubes are energized, a source of direct current potentials connected to the con-v f' trolV grid `of said amplifier tube, and a rectifier having its input electrodes coupled to said source of high frequency oscillations and its'output electrodescoupled to the control grid of said ther- V'mionic amplifier tube. to impress thereon a direct i current correction voltage the value of which is a function of the amplitude vof the high frequency oscillations of said source.

6. In a signalling system, a thermionic Vtube having an anode, a cathode and a control grid, a circuit for-applying carrier wave energy to the control grid and cathode of said tube, a source of'control potentials, acontrol tube having Aan anode, a cathode and a control grid, ya circuit couplingsaid source of control potentials to the control grid and cathode of said control tube,

means coupling the impedancebetween the anode and cathode of said control tube in a direct current circuit connected with the grid and cathode of said first named' tube to control the potential of the `grid of said first named tube Yrelative to the cathode of said vfirst named tube when' cur- Yrent flows in said direct current circuit, a supplecludes a source of directcurrent potential, theY Value of which varies in accordance With'the mean amplitude of the-control potentials.

FREDERICK CHARLES LUNNON. EDWARD HERBERT TRUMP. 

